1. Field of the Invention
This invention relates generally to satellite ranging system receivers.
2. Background Information
There are a number of satellite ranging systems that are currently deployed and additional systems are to be deployed in the near future. Each of these configurations is based upon transmission of ranging signals in particular frequency bands. More specifically, the present United States Global Positioning System (GPS) is based on transmission of ranging signals in two frequency bands known as L1, which is at a center frequency of 1575.42 MHz and L2, centered at 1227.6 MHz. To enhance the reliability and availability of this system, additional GPS signal structures are planned (e.g. L5, L2C). In addition, other satellite ranging systems are being deployed or have been deployed such as that of the Russian Federation, i.e., GLONASS (with two signal structures: G1 and G2), and the European GALLILEO system with multiple signal structures (referred to herein as: E1, E2 . . . E5, etc.)
The system satellites transmit precisely timed signals that contain a number of components, namely, a plurality of pseudo-random noise (PRN) codes and data. The signals allow for precise determination of latitude, longitude, elevation and time. The systems use a digital receiver, which receives PRN-encoded signals. A PRN ranging system receiver essentially synchronizes local versions of the transmitted codes to the received codes and operates by using time difference of arrival and Doppler measurement techniques to produce pseudoranges for the satellites then in view. The receiver then determines its global position using the pseudoranges.
Each satellite transmits on at least one carrier frequency that is modulated with low frequency (typically 50 hertz) digital data, which consists of information such as the satellite ephemeris, (i.e. position, current time of day, and system status information.) As noted, a satellite ranging signal receiver receives a composite signal consisting of one or more of the signals transmitted by the satellite within view (within a direct line of sight) as well as noise and interfering signals. Currently, there are 24 GPS satellites orbiting the Earth in 12-hour orbits. By determining the transmission time from at least four satellites and knowing each satellite's ephemeris, and approximate time of day information, the receiver can calculate the pseudoranges and thus its three-dimensional position, its velocity and the precise time of day.
A receiver typically downconverts the received satellite signal to an intermediate frequency (IF) signal before the signal is converted from an analog signal to a digital signal. At a downconverter stage, the input radio frequency (RF) signal from the antenna is first amplified and filtered and then down converted to the IF signal. The down conversion can be performed by “heterodyning,” which involves mixing the incoming signal with one or more locally generated carrier reference signals to produce the signal with a selected intermediate frequency (IF). This analog IF signal is then converted to digital samples by an A/D converter.
In a conventional receiver, there is a separate downconverter stage for each carrier frequency band of interest. Thus, in most typical designs, each respective downconverter stage includes at least one RF band pass filter component that filters out signals that are not within the desired frequency band, a heterodyning mixer, and an associated local oscillator that produces a carrier that has the frequency required to produce the desired IF signal after mixing with the received carrier. The downconverter stage further includes and various other amplifiers and filters. Coupled with these components is an A/D converter. Accordingly, in known designs, all of these components are needed for each frequency band signal structure being received and processed by the receiver. For example, one downconverter stage is needed for the L1 portion of an incoming GPS signal and a separate downconverter stage is required for the L2 portion of the incoming signal. This adds complexity and bulk to the electronic circuits of the GPS receiver.
As noted, there are several different satellite transmission systems and more are planned for the future. Each of these transmission systems transmits signals in two or more different frequency bands. Thus, in accordance with conventional designs, in order for a single receiver to obtain and use signals from multiple satellite systems, a separate downconverter stage has been required for the respective frequency bands. Thus, similar to the GPS environment just discussed, this duplication of circuitry adds to the cost, complexity and size of the receiver, as the circuitry requires greater area on integrated circuits in the receiver.
There remains a need, therefore, for a satellite ranging signal receiver of an architecture which is of an acceptable size and cost, and which accommodates multiple signal bands at a nominal increase in receiver complexity.
A GPS receiver may also incorporate a Controlled Reception Pattern Antenna (CRPA) for removal or processing of intentional and unintentional jamming signals. The CRPA includes a phased array that has multiple antenna elements, and information from each antenna element is used for forming the beam. At present a separate RF channel circuit is needed for obtaining and using the signal from each antenna element. The RF channels consist of filters, low noise amplifiers, mixers, variable controlled oscillators and AGC circuits. However, many such arrays include seven or more antenna elements. This results in a rather complex and large analog hardware section for an associated receiver for such a system.
There remains a need, therefore, a receiver architecture for use with a CRPA antenna, which uses less space than prior designs and involves a significantly reduced mutual coupling effect between antenna elements.